Heat shock proteins (Hsps) play dual roles in cellular biology; they are the first line of defense against cytotoxic stresses, and are tightly integrated into signaling and regulatory pathways under normal growth conditions. A subset of Hsps, including Hsp70 and HspQO, act as molecular chaperones, and may be required at multiple stages during a substrate protein's lifetime, ranging from biogenesis, localization, and stability, to activation and degradation. Protein chaperones are induced during numerous pathophysiological conditions including ischemia and tumorigenesis, and are known to facilitate stability and activity of oncoproteins such as v-src kinase and the p53 tumor suppressor. The Hsp110 class of chaperones is a poorly understood Hsp70 relative and is present in all eukaryotes, with tissue-specific isoforms of unknown function in humans. Our long-term objective is to elucidate the cellular roles of this chaperone family. The baker's yeast Hsp110 homolog is encoded by the SSE1 and SSE2 genes, and little is known about their function. We have discovered that Sse1 exists as a heterodimer in vivo with the cytosolic Hsp70s Ssa and Ssb, and that Sse1 is required for signal transduction activity of the Hsp90 chaperone system. Wetherefore hypothesize that Sse1, and by extension the mammalian Hsp110 chaperone, may function primarily as a modulator of Hsp70 activity. This proposal seeks to gain a mechanistic understanding of the cellular roles of Hsp110 chaperones by asking two specific questions: 1) How does Sse1 operate in partnership with the yeast Hsp70 Ssa1 and 2) How does Sse1 participate in signal transduction with Hsp90? In the first aim we will determine interaction sites and regulation of Hsp70 by Sse1 using purified chaperones, ultimately deciphering effects of Sse1 on protein folding in vitro. We will complement these experiments with in vivo assays to determine the contribution of Sse1 to Ssa1-dependent processes. In the second aim, we will determine both the stage at which Sse1 acts in the Hsp90 substrate folding cycle, and specific effects on substrate maturation using the model client protein glucocorticoid receptor. Finally we will apply these findings to understand how Sse1 in collaboration with Hsp90 is required for heat shock survival by modulating signaling through the Slt2 MAP kinase in the cell integrity pathway. These lines of investigation in yeast will serve as a model for predicting which processes Hsp110 may facilitate in mammalian cells.